Gel coated fiber reinforced plastics are subject to blistering if immersed in water for a prolonged time. Hulls of boats and walls of hot tubs are especially prone to blistering.
The blister problem costs boat owners enormous amounts of money each year. Further, the problem costs boat builders both money and loss of prestige.
The blistering of gel coated, glass reinforced plastic (GRP) structures such as boat hulls, swimming pools, bathroom components, etc. is a defect randomly affecting a portion of these products and is of major concern to manufactures and owners. This issue has been explored in discussions at ABBRA Meeting in Newport, R.I., Feb. 29, 1984; and in the following publications: Fraser-Harris, A. B. F. and Kyle, J. H., "FRP Bottom Blistering," Paper presented at the Chesapeake Sailing Yacht Symposium, Annapolis, Md. Jan. 15, 1983; Tuttle, B. L., "Safety Council Start Study of Boat Pox," Soundings, June, 1984; Norwood, L. S., Edgell, D. W. and Hankin, A. G., "Blister Performance of GRP Systems in Aqueous Environments," 36th Annual Conference SPI Reinforced Plastics/Composites Institute, Paper 23-F, (1981); Olford, S. C., "Osmosis: Causes and Effects," Yacht Brokers Designers and Surveyors Association Report (1978); Brueggeman, W. H., "Blistering of Gel Coat Laminates," 34th Annual Conference, SPI Reinforced Plastics/Composite Institute, Paper 4-E, (1979); and Anon, "Boatpox:Scratching the Surface of a Perplexing Problem," Practical Sailor, 10(15), (1984). Blisters appear as a raised, localized swelling of the gel coated laminate and occur after immersion. When blisters are ruptured a viscous acidic liquid is expelled.
It has been established that the diffusion of water into the composite and the presence of water-soluble constitutents within the laminate play important roles in blistering [Abeysinghe, H. P., Ghotra, J. S. and Pritchard, G., "Substances Contributing to the Generation of Osmotic Pressure in Resins and Laminates," Composites, 57 (1983), and Abeysinghe, H. P., Ghotra, J. S. Mahli, T. R. and Pritchard, G., "Blister Formation in RP: the Origin of the Osmotic Process," 38th Annual Conference, SPI Reinforced Plastics Composites Institute, Paper 17-B, (1983)].
The osmotic process plays an important role in blistering of the gel coat ["Substances Contributing to the Generation of Osmotic Pressure in Resins and Laminates," supra; "Blister Formation in RP: the Origin of the Osmotic Process," supra; and Ashbee, K. H. G., Frank, F. C. and Wyatt, R. C. "Water Damage in Polyester Resins," Proc. Roy. Soc., A300, (1967). In most polymers, water permeates into the laminate under a pressure or concentration gradient by a diffusion process which may be enhanced by surface cracks or pinholes in the gel coat. "Water Damage in Polyester Resins," supra, and Ashbee, K. H. G., Frank, F. C. and Wyatt, R. C. "Water Damage in Glass/Fibre Resin Composites," Proc. Roy. Soc. A312, (1969)]. Regester measured water permeation rates for various types of resins, [Regester, R. F., "Behavior of Fiber Reinforced Plastic Materials in Chemical Service," Corrosion, 25(4), (1969)]. Permeation rates have been shown to decrease as coating thickness increases [Hwang, S, and Kammenmeyer, K., "Effect of Thickness on Permeability," in Permeability of Plastic Films and Coatings to Gases, Vapors, and Liquids, Hoffenberg, H. B. (ed.), Polymer Science and Technology, Vol. 6, Plenum, N.Y. (1974)). Several studies also indicate certain polyester and epoxide resins show higher resistance to water permeation than general purpose orthophthalic resins ["Substances Contributing to the Generation of Osmotic Pressure in Resins and Laminates" supra; "Water Damage in Polyester Resin," supra; Abeysinghe, H. P., Edwards, W., Pritchard, G., and Swampillar, G. J. "Degradation of Crosslinked Resins in Water and Electrolyte Solutions," Polymer 23, (1982); and "Behavior of Fiber Reinforced Plastic Materials in Chemical Service", supra)]. Disk cracking has been shown to enhance permeation and epoxides show greater resistance to disk cracking than do some polyesters [Ghotra, J. S. and Pritchard, G., "Substances Contributing to the Generation of Osmotic Pressure in Resins and Laminates", supra] Specifically formulated polyurethane resins also have excellent resistance to water, [Paul, Swaraj, "Polyurethanes," in Surface Coatings--Science and Technology, Wiley, Chichester (1985)].
The susceptibility of the polymer to hydrolysis is also influenced by other components of the laminate. The laminate can contain easily hydrolyzable constituents or water-soluble residual substances, such as glass mat binder, pigment carriers, mold release agents, stabilizers, promoter components, catalyst components and uncrosslinked resin components. Studies have shown the rate and onset of blistering is determined by variations in laminate composition and fabrication procedures. ["Blister Performance of GRP Systems in Aqueous Environments", supra, "Blister Formation in RP: the Origin of the Osmotic Process," supra., "Variables Influencing the Blister Resistance of Marine Laminates", 37th Annual Conference, SPI Reinforced Plastics/ Composites Institute, Paper 21-B (1982) and Edwards, H. R., "Variables Influencing the Blister Performance of a Gel Coated Laminate," 34th Annual Conference SPI Reinforced Plastics/Composites Institute, Paper 4-D (1979)].
In order to prevent or reduce the incidence of blistering, it has been suggested that a barrier or protective coating be used over the polyester gel coat, [Ghotra, J. S. and Pritchard, G., "Osmotic Blister Formation and Prevention in FRP Marine Laminates," 28th National SAMPE Symposium (1983)]. The effectiveness of a barrier coating is determined by several factors. These include the coating composition, surface preparation, number of coats, application sequence of coating types, and compatibility of a multi-coat system which may include a primer.
The prior art technique has been to develop new gel coat formulations with improved hydrolytic stability. For example, an isophthalic acid-based resin has improved hydrolytic stability and thus is considered tto be less permeable than a general purpose orthophthalic acid-based resin. Formulations also include using neopentyl glycol (NPG) to replace the propylene glycol and other glycols in the polyester resin manufacturing processes. Due to the symmetry of NPG, it confers chemical stability by way of steric hindrance of the susceptible ester linkages to hydrolysis.
It is believed the blisters are caused by the formation of a solution inside the hull which grows by osmosis and as it expands, it pushes the overlying gel coat layer into a dome-shaped blister. Sizes can range from 1/2" to 4". The solution inside the blister forms by water diffusing slowly through the hull and being drawn into the osmotic center. The solution which forms in the blister is acidic, as well as under pressure, and can destroy the resin matrix by attacking the ester linkages. We placed three types of coatings over gel coated composites. The coating materials which are known to have a much lower water diffusivity than the general purpose orthophthalic acid-based polyester resin showed blistering and the more water diffusive polyester coating showed no blistering.
It appears this happens because an osmotic center is created as a result of low molecular weight hydrophilic molecules (which are always present in the material) concentrated at a spot. If the hull material is previous these low molecular weight species can be leached out of the fiber glass composite. When these constituents are absent, the osmotic process cannot occur and no osmotic pressure can build, therefore, no blisters will form.
Our invention is broadly directed to a composition and method wherein a gel coat is laminated to a fiber reinforced polymer and blistering of the gel coat is prevented when the fiber reinforced polymer is continually immersed in water. In a preferred embodiment a gel coated fiber reinforced polymer has a coating applied to the gel coat which prevents blistering.
Two coating materials were chosen to study the effect of a barrier coating on reducing water permeation into the gel coat polyester laminate. The coatings chosen were an epoxide and polyurethane. An orthophthalic acid-based resin was used as the criterion for evaluating blister formation. The epxoide and polyurethane resins are known for their chemical resistance, lower water diffusivity and hydrolytic stability.
It was discovered that the polyester resin, which is more permeable than the epoxide and polyurethane resins, prevented blistering.